Lithium secondary battery of improved high-temperature cycle life characteristics

ABSTRACT

Disclosed is a lithium secondary battery comprising a cathode including a lithium-containing transition metal oxide, and a non-aqueous electrolyte with addition of a compound of formula (1). Incorporation of the compound (1) into the electrolyte significantly improves the high-temperature performance and cycle life characteristics of the battery.

This application is a Continuation of co-pending Application Ser. No.13/040,832 filed on Mar. 4, 2011, which is a Continuation of ApplicationSer. No. 12/525,010 filed on July 29, 2009, now U.S. Pat. No. 7,923,157B2, which is the national phase of PCT International Application No.PCT/KR2007/006885 filed on Dec. 27, 2007, and which claims priority toApplication No. 10-2007-0012967 filed in the Republic of Korea on Feb.8, 2007. The entire content of all of the above applications is herebyexpressly incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a lithium secondary battery withimproved high-temperature cycle life characteristics. More specifically,the present invention relates to a lithium secondary battery withsignificantly improved high-temperature performance and cycle lifecharacteristics, comprising a cathode including a lithium-containingtransition metal oxide, an anode including a carbon-based material, anda non-aqueous electrolyte with the incorporation of a certain compoundwhich is capable of lowering a concentration of a hazardous materialcausing deterioration of the battery performance, through a chemicalreaction of that compound with such an undesirable material.Incorporation of such a certain compound into the electrolytesignificantly improves the high-temperature performance and cycle lifecharacteristics of the battery.

BACKGROUND OF THE INVENTION

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as anenergy source. Among other things, a great deal of research and studyhas been focused on lithium secondary batteries having high energydensity and voltage. These lithium secondary batteries are alsocommercially available and widely used.

In general, the lithium secondary battery is comprised of a cathode, ananode and a separator therebetween, with addition of an electrolyte. Inthis connection, the electrolyte is used in the form of a material whichcontains a suitable amount of a lithium salt dissolved in an organicsolvent. Examples of the lithium salt added to the electrolyte mayinclude conventionally used materials such as LiPF₆, LiBF₄, LiClO₄,LiN(C₂F₅SO₃)₂, and the like. These materials serve as a lithium ionsource in the battery to enable the basic operation of the lithiumbattery.

Currently available electrolytes undergo various side reactions duringcharge/discharge processes, and the thus-produced by-products may beresponsible for deterioration of the battery performance.

Particularly when a lithium salt LiPF₆ is incorporated into theelectrolyte, LiPF₆ should be present in ionic forms of Li⁺ and PF₆ ⁻.Usually, contrary to intentions, side reactions take place withproduction of an unstable by-product PF₅, which subsequently reacts withH₂O to result in formation of HF. HF causes destruction of the SEI layerand cathode dissolution, which becomes more severe at high temperatures.

Upon initial charge of the lithium secondary battery, lithium ionsdeintercalate from the cathode and intercalate between layers of thegraphite electrode used as the anode. At this time, the lithium ionsreact with carbon atoms of the anode to form a passivation film, calledSolid Electrolyte Interface (SET), on the anode surface. Once the SEIlayer is formed, the lithium ions do not undergo the side reaction withthe graphite anode or other materials. Therefore, destruction of the SEIlayer by HF resulting from the side reaction of LiPF₆ in the electrolytemay result in serious malfunction of the battery operation.

In order to prevent the problems as mentioned above, additives may beused in the electrolyte. A primary function of conventional electrolyteadditives was to prevent formation of by-products occurring upon chargeand discharge of the battery.

A conventional lithium secondary battery undergoes numerous sidereactions as well as a favorable forward reaction for the lithium saltLiPF₆, so the operation efficiency of the battery is lowered. Major sidereactions may include formation of LiF and PF₅ from decomposition ofLiPF₆ (side reaction-1), formation of HF and POF₃ from reaction of PF₅(from side reaction-1) with trace water in the electrolyte (sidereaction-2), and HF-induced SEI destruction on the anode (sidereaction-3).

Furthermore, additional materials other than HF may be produced such asHCl, HBr, and HI, depending upon kinds of lithium salts used as theelectrolyte. These by-products may act as acid thereby exertingpotentially deleterious functions as HF does.

In this connection, Korean Patent Application Publication No. 2006-92074A1, assigned to the present applicant, proposes a technique of improvingthe high-temperature storage performance of a battery via addition of anammonium compound to a non-aqueous electrolyte. The inventors of thepresent invention have made extensive investigations on such a subjectto improve the high-temperature storage performance of the battery. As aresult, they have found that when a certain compound of the presentinvention among these ammonium compounds is used in the fabrication ofthe battery, solubility of that compound in the electrolyte issignificantly increased to thereby greatly improve the high-temperaturestorage performance to a degree that cannot be achieved by the aboveKorean Patent. This fact can be further confirmed in Examples andComparative Examples which will be illustrated hereinafter. Meanwhile,even though it is unrelated to the present invention, U.S. Pat. No.4,535,389 discloses a technique of improving high-temperature outputcharacteristics via addition of ammonium benzoate to a capacitorelectrolyte. However, the above prior art is a technique which appliesto the capacitor. Further, ammonium benzoate, as described above, haslow solubility in an electrolyte for the secondary battery, and as suchit is impossible to improve the high-temperature storage performance toa desired degree.

Upon considering the fact that performance of a secondary battery athigh temperatures becomes more important, there is an urgent need fordevelopment of a more effective additive.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the inventors ofthe present invention, as will be described hereinafter, have discoveredthat, in a lithium secondary battery comprised of a cathode including alithium-containing transition metal oxide, an anode including acarbon-based material, a porous separator and a lithium salt-containingelectrolyte with addition of a certain compound which is capable oflowering a concentration of a hazardous material causing deteriorationof the battery performance, through a chemical reaction of that compoundwith such an undesirable material, the incorporation of such a compoundmakes it possible to fabricate a lithium secondary battery havingsignificantly improved high-temperature performance and cycle lifecharacteristics. The present invention has been completed based on thesefindings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with an aspect of the present invention, the above andother objects can be accomplished by the provision of a lithiumsecondary battery comprising:

(a) a cathode active material including a lithium-containing transitionmetal oxide having reversible lithium intercalation/deintercalationcapacity;

(b) an anode active material including graphitized carbon havingreversible lithium intercalation/deintercalation capacity;

(c) a porous separator; and

(d) a non-aqueous electrolyte containing (i) a lithium salt, (ii) anelectrolyte compound, and (iii) a compound of formula (1);

wherein R₁ is C₁-C₅ lower alkyl, and R₂ is hydrogen or C₁-C₅ loweralkyl.

The compound of formula (1) reacts with a material that contains acid(H) and then causes a side reaction in the electrolyte to thereby resultin deterioration of the battery performance. This reaction of thecompound (1) with such an undesirable material brings about conversionof the undesirable material into a non-reactive material, whichconsequently leads to formation of a stable surface coating on the anodeto thereby inhibit precipitation of metal ions. Therefore, it ispossible to suppress additional electrolyte decomposition resulting fromprecipitation of metal ions. In such a manner, addition of the compound(1) to the electrolyte can improve the high-temperature performance andcycle life characteristics of the battery, by reducing degradation ofhigh-temperature cycle characteristics of the lithium secondary batteryand deterioration of residual capacity and recovery capacity resultingfrom high-temperature storage of the battery.

Specifically in a secondary battery containing a lithium salt LiPF₆ as acathode active material, there are various side reactions as well as afavorable forward reaction as shown in the following reactions below, sothe operation efficiency of the battery is lowered. Occurrence of theseside reactions is more pronounced at high temperatures.

LiPF₆→Li⁺+PF₆ ⁻  (forward reaction)

LiPF₆→LiF+PF₅   (side reaction-1)

PF₅+H₂O→2HF+POF₃   (side reaction-2)

HF→destruction of anode SEI layer   (side reaction-3)

However, the present invention features the incorporation of thecompound (1) into the electrolyte, and the added compound (1) reactswith HF produced from the side reaction-1 and side reaction-2, whichbrings about conversion of acid having adverse effects on the batteryperformance into a non-reactive material to thereby inhibit occurrenceof the side reactions.

When R₁ and R₂ in formula (1) are independently C₁-C₅ lower alkyl, theterm “alkyl” refers to an aliphatic hydrocarbon group. An alkyl moietymay be a “saturated alkyl” group, thus representing that no alkene oralkyne portion is contained. Alternatively, the alkyl moiety may be an“unsaturated alkyl” group, thus being capable of containing at least onealkene or alkyne portion. The term “alkene” moiety refers to a group inwhich at least two carbon atoms form at least one carbon-carbon doublebond. The term “alkyne” moiety refers to a group in which at least twocarbon atoms form at least one carbon-carbon triple bond. The alkylmoiety, regardless of whether it is saturated or unsaturated, may bebranched, linear or cyclic. Examples of the alkyl may include, but arenot limited to, methyl, ethyl, propyl, iso-propyl, n-butyl, iso-butyl,t-butyl, pentyl, ethenyl, propenyl, butenyl, cyclopropyl, cyclobutyl,and cyclopentyl. Where appropriate, the alkyl group may also beoptionally substituted by hydroxy, halogen, amine, or the like.

The compound of formula (1) in accordance with the present invention, asdefined above, is characterized by substitution of R₁ at a meta positionof a benzene ring.

In this connection, according to the experiments conducted by thepresent inventors, it was confirmed that the compound of formula (1)exhibits significantly higher solubility in the electrolyte, as comparedto unsubstituted ammonium benzoate, or ortho- or para-substitutedanalogs. That is, when the ammonium compound with low solubility isused, a concentration of the added compound is variable, so it isdifficult to control physical properties of a desired product due tovariable and low effects on improvements of the high-temperatureperformance. Whereas, the meta-substituted ammonium compound as in thepresent invention has high solubility, and therefore exhibits excellentefficiency relative to an amount of the compound to be added, anduniformly improved high-temperature performance and cycle life.

Therefore, the compound of formula (1) has solubility of preferably atleast 1% by weight, more preferably 10% by weight or higher in theelectrolyte.

Further, it was confirmed that performance characteristics of thebattery are very excellent when R₁ is methyl. Accordingly, particularlypreferred is m-methylbenzoic acid methylammonium of formula (2) whereinR₁ and R₂ are independently methyl, or m-methylbenzoic acid ammonium offormula (3) wherein R₁ is methyl. The compound of formula (2) is morepreferable with respect to additive effects of the compound.

For example, the m-methylbenzoic acid methylammonium of formula (2)reacts with HF produced from side reactions in the electrolyte, therebyresulting in formation of m-methylbenzoic acid and methylammoniumfluoride in the form of a non-reactive material. Therefore, it ispossible to prevent deterioration of the battery performance and improvehigh-temperature cycle life characteristics of the battery.

The content of the compound (1) is preferably in a range of 0.01 to 10%by weight, based on the total weight of the electrolyte. When thecontent of the additive is too low, it is difficult to achieve desiredadditive effects. On the other hand, when the content of the additive istoo high, this undesirably leads to increased viscosity of theelectrolyte and increased resistance of the thus-fabricated battery,thereby deteriorating performance of the battery.

As discussed above, a lithium secondary battery in accordance with thepresent invention is comprised of a cathode active material including alithium-containing transition metal oxide, an anode active materialincluding a carbon-based material, a porous separator and a lithiumsalt, an electrolyte compound, and an electrolyte containing theabove-mentioned compound.

Examples of the lithium-containing transition metal oxide that can beused in the cathode active material may include one or more metal oxidesselected from the group consisting of compounds represented by formula(4):

LiCo_(a)Mn_(b)Ni_(c)M_(d)O₂   (4)

wherein

0≦a≦1;

0≦b≦1;

0≦c≦1;

0≦d≦1, with proviso that a+b+c+d=1; and

M is selected from the group consisting of Al, B, Ga, Mg, Si, Ca, Ti,Zn, Ge, Y, Zr, Sn, Sr, Ba and Nb; and

compounds represented by formula (5):

Li_(x)Mn₂—YM′_(Y)O₄   (5)

wherein

0.9≦X≦2;

0≦Y≦0.5; and

M′ is selected from the group consisting of Fe, Co, Ni, Cu, Zn, Al, Sn,Cr, V, Ti, Mg, Ca, Sr, B, Ga, In, Si and Ge.

Preferred examples of the compounds of formulae (4) and (5) may includeLiCoO₂, LiNiO₂, LiMn₂O₄, and LiNi_(1-x)Co_(x)O₂ (0<X<1).

The cathode is, for example, fabricated by applying a mixture of acathode active material, a conductive material and a binder to a cathodecurrent collector, followed by drying. If necessary, a filler may befurther added to the above mixture.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit to materials forthe cathode current collector, so long as they have high conductivitywithout causing chemical changes in the fabricated battery. As examplesof the materials for the cathode current collector, mention may be madeof stainless steel, aluminum, nickel, titanium, sintered carbon, andaluminum or stainless steel which was surface-treated with carbon,nickel, titanium or silver. The current collector may be fabricated tohave fine irregularities on the surface thereof so as to enhanceadhesion to the cathode active material. In addition, the currentcollector may take various forms including films, sheets, foils, nets,porous structures, foams and non-woven fabrics.

The conductive material is typically added in an amount of 1 to 50% byweight, based on the total weight of the mixture including the cathodeactive material. There is no particular limit to the conductivematerial, so long as it has suitable conductivity without causingchemical changes in the fabricated battery. As examples of conductivematerials, mention may be made of conductive materials, includinggraphite such as natural or artificial graphite; carbon blacks such ascarbon black, acetylene black, Ketjen black, channel black, furnaceblack, lamp black and thermal black; conductive fibers such as carbonfibers and metallic fibers; metallic powders such as carbon fluoridepowder, aluminum powder and nickel powder; conductive whiskers such aszinc oxide and potassium titanate; conductive metal oxides such astitanium oxide; and polyphenylene derivatives.

The binder is a component assisting in binding of the electrode activematerial with the conductive material, and in binding of the electrodeactive material with the current collector. The binder is typicallyadded in an amount of 1 to 50% by weight, based on the total weight ofthe mixture including the cathode active material. As examples of thebinder, mention may be made of polyvinylidene fluoride, polyvinylalcohols, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene butadiene rubber, fluoro rubber and variouscopolymers.

The filler is an optional ingredient used to inhibit cathode expansion.There is no particular limit to the filler, so long as it does not causechemical changes in the fabricated battery and is a fibrous material. Asexamples of the filler, there may be used olefin polymers such aspolyethylene and polypropylene; and fibrous materials such as glassfiber and carbon fiber.

As the carbon-based material in the anode active material, it ispreferred to use graphitized carbon in which a carbonaceous materialused as the anode active material has the lattice spacing (d₀₀₂) of lessthan 0.338 nm, as measured by X-ray diffraction, and has a specificsurface area of less than 10 m²/g, as measured by a BET method.

The anode is fabricated by applying an anode material to an anodecurrent collector, followed by drying. If necessary, other components asdescribed above may be further included.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit to materials for the anodecurrent collector, so long as they have suitable conductivity withoutcausing chemical changes in the fabricated battery. As examples ofmaterials for the anode current collector, mention may be made ofcopper, stainless steel, aluminum, nickel, titanium, sintered carbon,copper or stainless steel having a surface treated with carbon, nickel,titanium or silver, and aluminum-cadmium alloys. Similar to the cathodecurrent collector, the anode current collector may also be processed toform fine irregularities on the surfaces thereof so as to enhanceadhesion to the anode active material. In addition, the anode currentcollector may be used in various forms including films, sheets, foils,nets, porous structures, foams and non-woven fabrics.

The separator is interposed between the cathode and anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or a glass fiber or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

The non-aqueous electrolyte for a lithium secondary battery is composedof a non-aqueous electrolyte and a lithium salt. As the non-aqueouselectrolyte, a non-aqueous electrolytic solution, an organic solidelectrolyte or an inorganic solid electrolyte may be utilized.

As examples of the non-aqueous electrolytic solution that can be used inthe present invention, mention may be made of aprotic organic solventssuch as N-methyl-2-pyrollidinone (NMP), propylene carbonate (PC),ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate(DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC),gamma-butyro lactone (GBL), 1,2-dimethoxy ethane, tetrahydroxy Franc,2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate. Among these solvent compounds, particularly preferred areethylene carbonate (EC), propylene carbonate (PC), butylene carbonate(BC), vinylene carbonate (VC), diethyl carbonate (DEC), dimethylcarbonate (DMC), ethylmethyl carbonate (EMC), gamma-butyro lactone(GBL), sulfolane, methyl acetate (MA), ethyl acetate (EA), methylpropionate (MP) and ethyl propionate (EP).

In one preferred embodiment of the present invention, the non-aqueouselectrolytic solution may be a mixture of at least one linear carbonatecompound and at least one cyclic carbonate compound. Representativeexamples of the linear carbonate may include dimethyl carbonate (DMC),diethyl carbonate (DEC) and ethylmethyl carbonate (EMC). As examples ofthe cyclic carbonate, mention may be made of ethylene carbonate (EC),propylene carbonate (PC) and butylene carbonate (BC). Particularlypreferred is a mixed non-aqueous electrolytic solution of EC/EMC.

As examples of the organic solid electrolyte utilized in the presentinvention, mention may be made of polyethylene derivatives, polyethyleneoxide derivatives, polypropylene oxide derivatives, phosphoric acidester polymers, poly agitation lysine, polyester sulfide, polyvinylalcohols, polyvinylidene fluoride, and polymers containing ionicdissociation groups.

As examples of the inorganic solid electrolyte utilized in the presentinvention, mention may be made of nitrides, halides and sulfates oflithium such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄,LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH andLi₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and may include, for example,LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆,LiSbF₆, LiA1C1₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroborane lithium,lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate andimide.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further include halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may additionally include carbon dioxide gas.

The secondary battery in accordance with the present invention may bepreferably used for a high-power, large-capacity, medium/large batterymodule, via a combination of multiple batteries as a unit battery. Thisis because the high-temperature cycle life may be an important factorwhich is necessary to exert desired operation characteristics, as thehigh-power, large-capacity, medium/large battery module is frequentlysusceptible to external forces such as vibration, external impact, etc.and therefore requires excellent mechanical strength against externalforces, and a loading amount of an electrode active material relative toa current collector is high in the structure of a battery cellconstituting a battery pack.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Example 1

0.1% by weight of m-methylbenzoic acid methylammonium was added to asolution of 1M LiPF₆ in ethylene carbonate/ethyl methyl carbonate(hereinafter, referred to as a 1M LiPF₆ EC/EMC solution) and the mixturewas then stirred to prepare an electrolyte.

Example 2

0.2% by weight of m-methylbenzoic acid methylammonium was added to a 1MLiPF₆ EC/EMC solution and the mixture was then stirred to prepare anelectrolyte.

Example 3

0.5% by weight of m-methylbenzoic acid methylammonium was added to a 1MLiPF₆ EC/EMC solution and the mixture was then stirred to prepare anelectrolyte.

Example 4

0.1% by weight of m-methylbenzoic acid ammonium was added to a 1M LiPF₆EC/EMC solution and the mixture was then stirred to prepare anelectrolyte.

Comparative Example 1

A 1M LiPF₆ EC/EMC solution without addition of m-methylbenzoic acidmethylammonium was used as an electrolyte.

Comparative Example 2

0.1% by weight of ammonium benzoate was added to a 1M LiPF₆ EC/EMCsolution and the mixture was then stirred to prepare an electrolyte.

Comparative Example 3

0.1% by weight of o-methylbenzoic acid methylammonium was added to a 1MLiPF₆ EC/EMC solution and the mixture was then stirred to prepare anelectrolyte.

Comparative Example 4

0.1% by weight of o-methylbenzoic acid ammonium was added to a 1M LiPF₆EC/EMC solution and the mixture was then stirred to prepare anelectrolyte.

Comparative Example 5

0.1% by weight of p-methylbenzoic acid methylammonium was added to a 1MLiPF₆ EC/EMC solution and the mixture was then stirred to prepare anelectrolyte.

Comparative Example 6

0.1% by weight of p-methylbenzoic acid ammonium was added to a 1M LiPF₆EC/EMC solution and the mixture was then stirred to prepare anelectrolyte.

Experimental Example 1

The solubility of additives in an electrolyte was measured for Examples1 and 4, and Comparative Examples 2 to 6. The results obtained are givenin Table 1 below.

TABLE 1 Additives Chemical structure Solubility Ex. 1 m-methylbenzoicacid methylammonium

10 wt % or higher Ex. 4 m-methylbenzoic acid ammonium

10 wt % or hight Comp. Ex. 2 Ammonium benzoate

0.03 wt % or less Comp. Ex. 3 o-methylbenzoic acid methylammonium

0.75 wt % or less Comp. Ex. 4 o-methylbenzoic acid ammonium

0.75 wt % or less Comp. Ex. 5 p-methylbenzoic acid methylammonium

0.03 wt % or less Comp. Ex. 6 p-methylbenzoic acid ammonium

0.03 wt % or less

As shown in Table 1, m-methylbenzoic acid methylammonium andm-methylbenzoic acid ammonium according to Examples 1 and 4 of thepresent invention exhibited high solubility of more than 10% by weight,whereas ammonium benzoate and para-methyl-substituted analogs exhibitedextremely low solubility of less than 0.03% by weight, thus representingthat they are substantially insoluble. Further, it can be seen thatortho-methyl-substituted analogs exhibit low solubility of less thanapproximately 0.75% by weight, ranging from 0.5 to 1.0% by weight.Therefore, when the additive compound has low solubility, aconcentration of the added compound is variable, so there is difficultywith control of physical properties and desired improvement of thehigh-temperature performance.

Experimental Example 2

Using spinel manganese as a cathode, a carbon electrode as an anode, andelectrolytes prepared in Examples 1 through 3 and Comparative Examples 1and 2, the corresponding pouch-type full cells were fabricated. The-thusfabricated cells were stored at a temperature of 60° C. for 8 weeks andwere measured for the output (in terms of a ratio relative to initialoutput) after 4 and 8 weeks, respectively. The results obtained aregiven in Table 2 below.

TABLE 2 Output (%) after high-temperature storage After 4 weeks After 8weeks Ex. 1 94.4 90.2 Ex. 2 95.0 92.5 Ex. 3 95.3 93.6 Comp. Ex. 1 91.782.2 Comp. Ex. 2 93.6 88.4

As can be seen from Table 2, the lithium secondary batteries withaddition of a certain compound (m-methylbenzoic acid methylammonium)capable of lowering a concentration of a material causing deteriorationof the battery performance through the chemical reaction of thatcompound with such an undesirable material to the electrolyte of thepresent invention (Examples 1 to 3) exhibited a significantly excellentoutput ratio relative to the initial output after high-temperaturestorage, as compared to the lithium secondary battery to which noadditive was added (Comparative Example 1). That is, it was confirmedthat incorporation of the m-methylbenzoic acid methylammonium leads toinhibition of side reactions within the battery during high temperaturestorage, thereby significantly improving capacity characteristics of thebattery. Further, high-temperature storage characteristics are improvedwith increasing contents of m-methylbenzoic acid methylammonium and alonger period of high-temperature storage time.

The lithium secondary battery (Comparative Example 2) with addition ofammonium benzoate, as shown in Table 2, exhibited high-temperaturestorage characteristics to some extent, even though it is inferior tothe secondary batteries of Examples 1 to 3 in accordance with thepresent invention. Therefore, in order to confirm the applicationreliability upon practical fabrication of the battery, 30 secondarybatteries of Example 1 and 30 secondary batteries of Comparative Example2 were fabricated and measured for high-temperature storagecharacteristics under the same conditions as described above.

As a result, the secondary batteries of Example 1 exhibited an outputdeviation of less than 1.5% based on the mean value, afterhigh-temperature storage for 8 weeks, whereas the secondary batteries ofComparative Example 2 exhibited an output deviation of about 5% underthe same conditions. The reason why the secondary batteries ofComparative Example 2 exhibited a relatively high output deviation isbelieved to be probably due to the practical concentration of theadditive in the electrolyte not being maintained at a constant level,due to poor solubility of the additive in the electrolyte. Accordingly,it can be seen that the secondary battery of the present invention issignificantly superior in terms of the application reliability uponpractical fabrication of a secondary battery.

INDUSTRIAL APPLICABILITY

As apparent from the above description, a lithium secondary battery inaccordance with the present invention can realize improvedhigh-temperature performance and cycle life characteristics, via theincorporation of a certain compound which chemically reacts with amaterial causing deterioration of the battery performance to therebylower a concentration of such an undesirable material.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A lithium secondary battery comprising: (a) a cathode active materialincluding a lithium-containing transition metal oxide having reversiblelithium intercalation/deintercalation capacity; (b) a porous separator;and (c) a non-aqueous electrolyte containing (i) a lithium salt, (ii) anelectrolyte compound, and (iii) a compound of formula (1);

wherein R₁ is C₁-C₅ lower alkyl, and R₂ is hydrogen or C₁-C₅ loweralkyl.
 2. The lithium secondary battery according to claim 1, wherein R₁is methyl.
 3. The lithium secondary battery according to claim 1,wherein R₂ is methyl.
 4. The lithium secondary battery according toclaim 1, wherein the compound of formula (1) has solubility of 1% byweight or higher in the electrolyte.
 5. The lithium secondary batteryaccording to claim 4, wherein the compound of formula (1) has solubilityof 10% by weight or higher in the electrolyte.
 6. The lithium secondarybattery according to claim 1, wherein a content of the compound (1) isin the range of 0.01 to 10% by weight, based on the total weight of theelectrolyte.
 7. The lithium secondary battery according to claim 1,wherein the lithium-containing transition metal oxide is selected fromthe group consisting of LiCoO₂, LiNiO₂, LiCo _(1/3)Mn_(1/3)Ni_(1/3)O₂,LiCo_(0.2)Mn_(0.3)Ni_(0.5)O₂, LiCo_(0.2)Mn_(0.4)Ni_(0.4)O₂, LiMn₂O₄ andLiNi_(1-x)Co_(x)O₂ (0<X<1).
 8. The lithium secondary battery accordingto claim 1, wherein the carbon-based material is graphitized carbonhaving the lattice spacing (d₀₀₂) of less than 0.338 nm, as measured byX-ray diffraction, and a specific surface area of less than 10 m²/g, asmeasured by a BET method.
 9. The lithium secondary battery according toclaim 1, wherein the lithium salt is selected from the group consistingof LiClO₄, LiCF₃SO₃, LiC₄F₉SO₃, LiPF₆, LiBF₄, LiAsF₆, LiN(C₂F₅S0₂)₂ andLiN(CF₃SO₂)₂.
 10. The lithium secondary battery according to claim 1,wherein the electrolyte compound is at least one selected from the groupconsisting of ethylene carbonate (EC), propylene carbonate (PC),butylene carbonate (BC), vinylene carbonate (VC), diethyl carbonate(DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC),gamma-butyro lactone (GBL), sulfolane, methyl acetate (MA), ethylacetate (EA), methyl propionate (MP), ethyl propionate (EP), and anycombination thereof.
 11. A battery module comprising the lithiumsecondary battery of claim 1, as a unit battery.
 12. The lithiumsecondary battery according to claim 1, wherein the lithium-containingtransition metal oxide is at least one selected from the groupconsisting of LiCoO₂, LiNiO₂, LiMnO₂, LiCo_(x)Mn_(y)Ni_(z)O₂ (0<x<0.4,0<y<0.5, 0<z<0.7), LiMn₂O₄ and LiNi_(1-x)Co_(x)O₂ (0<X<1).